As is well known, a differential is a gear assembly in a motor vehicle which allows the drive shaft to turn the drive wheels at different speeds when the vehicle is going around a curve. When a vehicle goes around a curve, the wheel on the inside of the curve travels less distance than the other, and so must turn more slowly, for safety in handling and to keep tire wear to a minimum. Some units are designed to give a limited-slip or slip-lock differential, to equalize power between the wheels on a slippery or a soft road surface, providing safe handling and minimizing the likelihood of getting stuck in snow or soft earth.
Automotive differentials are positioned within drive lines for dividing engine drive power between two output shafts. Front and rear differentials respectively divide the engine power between the axle halves of front and rear axles, and center differentials divide the engine power between drive shafts to the front and rear axles.
A planetary gear set mounted within a differential housing interconnects the two output shafts for rotation in opposite directions with respect to the housing (i.e., differentiation). An input shaft delivers engine power to the housing for rotating the housing together with the planetary gear set about a common axis of the pair of output shafts.
Sun gear members of the planetary gear set, also referred to as "side gears", are coupled to inner ends of the output shafts. Planet gear members of the same set are positioned within the housing for transmitting power between the sun gears. The planet gear members may be arranged in a "cross-axis" orientation with respect to the axes of the sun gears, or in a "parallel-axis" orientation, where the axes of the planet gears are parallel to the coincident axes of the side gears and output shafts. In a parallel-axis configuration, one portion of each planet gear meshes with one of the side gears, and another portion of each planet gear meshes with its paired planet gear.
Commonly assigned U.S. Pat. No. 5,122,101 to Tseng discloses a parallel-axis gear differential in which the planet gears are formed as so-called "combination" gears having main and transfer gear sections separated by a stem. The main gear section meshes both with one of the two side gears and with the transfer section of a paired combination gear. The transfer gear section meshes with the main gear section of the paired combination gear. The two meshes between paired combination gears straddle two meshes between the paired combination gears and the side gears.
Another known parallel-axis gear differential includes one combination gear member of each planet gear pair. The main gear section of the one combination gear member meshes with one of the two side gears, and the transfer gear section meshes with its paired planet gear. The single mesh between the paired planet gears overlaps one side of the two planet gear-to-side gear meshes.
The planet gears can be supported for rotation on shafts or within pockets formed in the housing. The shafts are received within bores that are also formed in the housing. The pockets provide bearings for supporting outside cylinder surfaces of the planet gears including top lands of the planet gear teeth. U.S. Pat. No. 5,244,440 to Ichiki et al., which is also commonly assigned, discloses such pockets as well as gearing relationships for maintaining preferred gear running positions within the pockets.
My grandparent Application No. 08/058,480 discloses an alternative gear mounting system in which the planet gears are supported in pairs between pedestals. Each pedestal has two gear mounting surfaces for supporting one member from each of two adjacent pairs of planet gears. Gear reactionary forces can be transmitted between the pairs of planet gears by mounting the pedestals on pivots.
The planetary gearing interacts with its mounting surfaces to produce frictional torque that supports uneven distributions of drive torque between the two output shafts. The frictional torque opposes relative rotation between the output shafts (i.e., differentiation) proportional to a drive torque applied to the housing. Accordingly, drive torque is divided between relatively rotating output shafts in accordance with a so-called "bias ratio", which is expressed as a normalized ratio of the torque in the output shaft receiving more torque divided by the torque in the output shaft receiving less torque.
The resistance to differentiation can compensate for uneven amounts of traction available to a pair of drive wheels. For example, a bias ratio of 2:1 can distribute two times more torque to one drive wheel of a pair having higher traction than the other. This prevents the lower traction drive wheel from spinning with respect to its traction surface and provides for the delivery of more total torque.
The frictional torque opposing relative rotations of the output shafts is composed of a series of frictional torques developed at different frictional interfaces throughout the differential. However, patterns of loading at the frictional interfaces vary depending upon directions of torque transfer through the differential. For example, the loading patterns vary between forward drive loading and reverse drive loading. Opposite directions of differentiation also change loading patterns.
Different bias ratios can result from the different loading patterns. In some instances, such different bias ratios are desirable; and in other instances, they are undesirable. However, even when different bias ratios are desirable, each bias ratio can still have a preferred value. For example, one bias ratio can be preferred for forward drive loading and another bias ratio can be preferred for reverse drive loading. Different bias ratios can also be preferred in center differentials for independently controlling the percentages of torque that can be distributed to the front and rear axles during opposite directions of differentiation. In contrast, only one bias ratio is generally preferred for opposite directions of differentiation in front and rear differentials.
In conventional differentials, torque bias ratios (TBRs) are largely a function of design structure. For example, cross-axis differentials typically demonstrate TBRs of 2.5:1 to 4:1 while parallel-axis differentials typically demonstrate TBRs of 1:1 to 2.5:1.
In most differentials, only limited possibilities exist for independently controlling bias ratios in the different directions of torque transfer. Most attempts at controlling bias ratios have thus far involved varying coefficients of friction between interfaces that are loaded differently between two or more directions of torque transfer. For example, co-assigned U.S. Pat. No. 4,890,511 to Pederson uses different coefficients of friction on opposite sides of a stationary washer to influence bias ratios in opposite directions of differentiation. Another co-assigned U.S. Pat. No. 5,232,415 to Brewer et al. uses different coefficients of friction at planet gear end faces to influence bias ratios both between opposite directions of differentiation and between forward and reverse drive.
In my parent Application No. 08/282,622, I disclose a differential which uses unique movable gear mounting surfaces to control bias ratios between forward and reverse loading, as well as variations in the gearing configuration with respect to the movable mounting surfaces which can be used to control bias ratios associated with opposite directions of differentiation. In this invention, pivoting pedestals function to transfer gear reactionary forces between following gear members and leading gear members.
A differential which can be controlled would be desirable and beneficial in vehicle handling. It would have a reduced TBR range (i.e., 2:1 to 2.5:1) for normal driving conditions, and an increased TBR range (i.e., 2.5:1 to 4:1) for greater traction during maneuvers or on variable road surfaces. For extreme differences in traction, the differential would lock. What is needed, then, is a differential which provides a fine control of TBR, and a differential whose TBR can be controlled by external means (i.e., an "active" differential).